$87 for $100 since it's 25% off 1 card and not total. You get $13 free, $26 free if you spent $174 etc.

It's better if you buy in bulk. I bought $400 worth, 4 $50 PSN and 4 $50 XBONE. I get $52, almost a full retail game. I went full digital this year and with about 6-7 games coming out between this month (diablo) till November i save money by doing this. If you don't buy digitally it's not worth the money.

I don't do the math per search but you can make $25 a month as a silver member and $50 a month as a gold member. I also know of a way you can make $20 in 8 days with Bing only if you a new member. This is great for people that only want to a buy dlc

This is basically 12.5% off because at a minimum you have to pay $25 for one card and (assuming you pay no more than $25) $18.75 for another. In total you pay $43.75 for $50 worth of credit. Not a terrible deal as long as you're optimistic. Find a $50 game on sale for $35 and you make $8 bucks off the deal.